Photoelectrochemical Water Splitting

Transcription

1 Electronic Supplementary Information (ESI) ZnO-TiO 2 Core-Shell Nanowires: A Sustainable Photoanode for Enhanced Photoelectrochemical Water Splitting Kyuwon Jeong, a# Prashant R. Deshmukh, a# Jinse Park, a Youngku Sohn, b Weon Gyu Shin a* a Department of Mechanical Engineering, Chungnam National University, Daejeon-34134, Republic of Korea b Department of Chemistry, Chungnam National University, Daejeon-34134, Republic of Korea *Corresponding Author: Tel.: ; Fax: wgshin@cnu.ac.kr (W.G. Shin) Number of Pages: S1-S14 List of Figures: Figure S1. Schematic representation of CVD method for the synthesis of (a) ZnO NWs and (b) ZnO-TiO 2 core-shell NWs heterostructure. Figure S2. FE-SEM (a-d) top view (inset is the magnified view) and (e-h) crosssection (tilted view) images of ZnO nanostructures grown at 550, 600, 650, and 750 C temperatures with constant carrier gas flow rate of 0.6 LPM. Figure S3. FE-SEM (a-c) top view (inset is the magnified view) and (d-f) crosssection (tilted view) images of ZnO nanostructures grown at 0.5, 0.7 and 1 LPM carrier gas flow rates with constant temperature of 700 C. Figure S4. FE-SEM (a-b) top view and (c-d) cross-section (tilted view) images of ZnO NRs and film obtained at 5 and 30 min., respectively at constant 700 C temperature and 0.6 LPM carrier gas flow rate. Figure S5. (a) HR-TEM image and (b) SAED pattern of ZnO NW. S1

2 Figure S6. XPS results: (a) Wide-scan spectra of ZnO NWs and ZnO-TiO 2 core-shell NWs. Fitted XPS spectra of O1s peak: (b) ZnO NWs and (c) ZnO-TiO 2 core-shell NWs. Figure S7. Photocurrent density of ZnO nanostructures synthesized: (a) at various temperatures ranging from C with constant carrier gas flow rate of 0.6 LPM and (b) at various carrier gas flow rates of LPM with constant temperature of 700 C. Figure S8. Mott Schottky plots of ZnO NWs and ZnO-TiO 2 core-shell NWs at a 5 khz constant frequency. List of Tables: Table S1. Experimental Parameters for the Synthesis of ZnO Nanowires. Table S2. PEC Performance of ZnO based Core-Shell Nanostructured Photoelectrodes for Water Splitting Application S2

3 Experimental Method Figure S1 shows the schematic representation of CVD method used for the synthesis of ZnO NWs and ZnO-TiO 2 core-shell NWs heterostructure. Typically, CVD method comprised of tube furnace, alumina tube, mass flow controller (MFC) and alumina boat as shown in Figure S1. Tube furnace is digital electric furnace designed by local company Sion Science, South Korea. Brooks MFC 5850E, Japan made mass flow controller was used. The alumina tube has 68 cm length and 2 cm outer diameter (1.5 cm inner diameter). The alumina boat has 5 cm length, 1 cm width, and 0.9 cm height. Figure S1. Schematic representation of CVD method for the synthesis of (a) ZnO NWs and (b) ZnO-TiO 2 core-shell NWs heterostructure. Material Characterizations S3

4 Surface morphology was observed by field emission scanning electron microscope (FE-SEM, JEOL, JSM-7000F). The crystallinity of the prepared samples was characterized by X-ray diffraction (XRD) with D8 Discover diffractometer (Bruker AXS, Germany), using CuKα (λ =1.54 Å) and high-resolution transmission electron microscope (HR-TEM, FEI, Tecnai G2 F30, Thermo Fisher Scientific). Raman spectroscopy and Photoluminescence (PL) was carried out by LabRAM HR- 800 (HORIBA JOBIN YVON). PL measurement of the samples was made with the excitation wavelength of 514 nm. UV-vis absorbance was characterized by UV-VIS- NIR Spectrophotometer (SHIMADZU, SolidSpec-3700). The elemental composition was analysed by X-ray photoelectron spectroscopy (XPS), (MultiLab 2000, Thermo Fisher Scientific). Photoelectrochemical Characterization The photoelectrochemical study of photoelectrodes was investigated by using the three-electrode system in the 0.5 M electrolyte (ph-7), where photoelectrode, platinum, and Ag/AgCl were used as a working photoanode, counter and reference electrodes, respectively. All the electrodes were connected to the external circuit by means of small crocodile pins. Furthermore, we ensured that there is no contact between the crocodile pins and electrolyte. The photocurrent was measured under an irradiation from 300 W Xenon lamp (XENON SHORT ARC LAMP, USHIO, JAPAN). The intensity of the light source was calibrated with Sireference cell (Model 15151, ABET Technologies) to simulate AM 1.5 G illumination (100 mwcm -2 ). The actual area of the photoanode exposed to the light was 0.5 cm X 0.5 cm. Linear sweep voltammetry (LSV) was carried out at the scan rate of 10 mvs -1 S4

5 in the potential range of 0.5 to +1 V vs. Ag/AgCl. All the potential was converted to the reversible hydrogen electrode (RHE) using the following equation [6]; E RHE = EAg / AgCl + EAg / AgCl pH where EAg / AgCl = V at 25 C (1) All the photoelectrochemical measurement was performed by potentiostat (IVIUM Technology) in the dark and simulated solar light. The water splitting efficiency of ZnO NWs and ZnO-TiO 2 core-shell NWs were calculated using the following equation [43]; 0 ( 1.23 E )/ I where E = E + E 0. ph η (%) = J (2) RHE light RHE Ag / AgCl Ag / AgCl where, η is water-splitting efficiency, J is photocurrent density (macm -2 ), E RHE is standard reversible hydrogen electrode potential, which is 1.23, and I light is intensity of solar light (mwcm -2 ). Experimental Parameters for the Synthesis of Vertically Aligned ZnO Nanowires Table S1. Experimental parameters for the synthesis of ZnO nanowires. Parameters Flow Rate of Carrier Gas (LPM) Temperature ( C) Time (sec.) Temperature Flow Rate S5

6 Time Effect of Temperature on the Surface Morphology of ZnO S6

7 Figure S2. FE-SEM (a-d) top view (inset is the magnified view), and (e-h) crosssection (tilted view) images of ZnO nanostructures grown at 550, 600, 650, and 750 C temperatures with constant carrier gas flow rate of 0.6 LPM. Effect of Carrier Gas Flow Rate on the Surface Morphology of ZnO S7

8 Figure S3. FE-SEM (a-c) top view (inset is the magnified view) and (d-f) crosssection (tilted view) images of ZnO nanostructures grown at 0.5, 0.7 and 1 LPM carrier gas flow rates with constant temperature of 700 C. Effect of Time on the Surface Morphology of ZnO S8

9 We have observed the effect of synthesis time on the ZnO nanostructures. It is found that the ZnO NWs obtained at 30 sec. synthesis time are changed into the NRs and thin films at 5 and 30 min. synthesis time, respectively. Figure S4 shows the FE- SEM images (top & tilted view) of ZnO NRs and thin film (See the Table S1 for detail conditions). ZnO NRs have the length of 10 µm and diameter of 500 nm (Figure S4 (a & c)). ZnO film thickness is about µm (Figure S4 (b & d)). There is no significant performance of ZnO NRs and thin films synthesized at 5 and 30 min., respectively, in the PEC water splitting as compared to NWs; therefore, further structural and PEC study have not considered. Figure S4. FE-SEM (a-b) top view and (c-d) cross-section (tilted view) images of ZnO NRs and film obtained at 5 and 30 min., respectively at constant 700 C temperature and 0.6 LPM carrier gas flow rate. HR-TEM Image and SAED Pattern of ZnO NW S9

10 Figure S5 (a & b) shows the HR-TEM image and SAED (selected area electron diffraction) pattern, respectively, of vertically aligned ZnO NW obtained at 700 C, 0.6 LPM carrier gas flow rate and synthesis time of 30 seconds. Figure S5 (a & b) shows a high-resolution transmission electron microscope (HR-TEM) image and the selected area electron diffraction (SAED) pattern of single ZnO NW, respectively indicating the crystalline nature of ZnO NW. The inter plane distance of 0.26 nm observed from Figure S5a corresponds to the lattice spacing of (002) crystal planes of ZnO indicating the progress of ZnO NW along the (001) direction. Figure S5. (a) HR-TEM image and (b) SAED pattern of ZnO NW. XPS Results of ZnO and ZnO-TiO 2 Core-Shell NWs S10

11 Figure S6. XPS results: (a) Wide-scan spectra of ZnO NWs and ZnO-TiO 2 core-shell NWs. Fitted XPS spectra of O1s peak: (b) ZnO NWs and (c) ZnO-TiO 2 core-shell NWs. Photoelectrochemical Water Splitting Results of ZnO Nanostructures S11

12 Figure S7. Photocurrent density of ZnO nanostructures synthesized: (a) at various temperatures ranging from C with constant carrier gas flow rate of 0.6 LPM, and (b) at various carrier gas flow rates of LPM with constant temperature of 700 C. Mott Schottky plots The capacitance of ZnO NWs and ZnO-TiO 2 core-shell NWs heterostructure in S12

13 the electrolyte solution was measured by EIS performed at 5 khz in the dark together with Mott-Schottky. This is used to determine the flat band potential at semiconductor/liquid electrolyte contacts. Figure S8 shows the Mott-Schottky plots of (a) ZnO NWs and (b) ZnO-TiO 2 core-shell NWs. The flat band potential of ZnO NWs and ZnO-TiO 2 core-shell NWs can be estimated using the Mott-Schottky equation [48, 72]; 1 C 2 = 2 e εε N 0 0 D ( V V ) FB kt e o (3) Where, e 0 is the electron charge, ε is the dielectric constant of ZnO, ε 0 is the permittivity of vacuum, N D is donor density, V is the potential difference across the semiconductor space charge region, V FB the flat band potential, and kt/e 0 is a temperature-dependent correction term. 7 6 ZnO NWs ZnO-TiO 2 core-shell NWs 1/C 2 (10 15, F 2 cm 4 ) Potential (V vs Ag/AgCl) Figure S8. Mott Schottky plots of ZnO NWs and ZnO-TiO 2 core-shell NWs at a 5 khz constant frequency. S13

14 Table S2: PEC Performance of ZnO based Core-Shell Nanostructured Photoelectrodes for Water Splitting Application Table S2. PEC Performance of ZnO based Core-Shell Nanostructured Photoelectrodes for Water Splitting Application Photocurrent Photoelectrodes Method Electrolyte ph Density (macm -2 Efficiency (%) Ref. ) Au decorated/3d B- ZnO NWs Match like ZnO/Au Plasmonic Heterostructure Ionic layer adsorption along with thermal-reduction and Hydrothermal Hydrothermal and Photochemical 0.5 M 0.1 M ~ at at +1.0 V 0.52 at ~ at ZnO NRs/BiVO 4 CBD and SILAR 0.5 M at +1.2 V - 49 ZnO/Fe 2 O 3 Core Shell Hydrothermal and Spin 1 M NWs Coating NaOH at +0.6 V - 48 ZnO ZnGa 2 O 4 Core 0.1 M CVD and CVD Shell NWs at +0.7 V - 42 Si/ZnO Core/Shell Metal Assisted Electroless 0.5 M at at +2.0 V NWAs Etching and ALD Na 2 SO SCE 4 V SCE 10 Si/ZnO Core/Shell Metal Assisted Electroless 0.5 M 0.47 at V 7 - (20mWcm 2 ) NWAs Etching and Sol-Gel NaClO 4 50 ZnO@CoNi LDH Hydrothermal and 0.5 M Core Shell NWAs Electrosynthesis at +0.5 V - 40 Electrochemical and successive ZnO:Cl-TiO 2 Core 0.1 M adsorption-hydrolysiscondensation steps 2 SO 4 Shell NWs Na - < at V - 54 ZnO-TiO 2 NTs CBD and Electrochemical 1 M anodization NaOH at V - 39 ZnO-TiO 2 Core-Shell Hydrothermal and Sol-gel 0.1 M at at S13

15 NWs NaOH V ZnO-TiO 2 Core-Shell 0.1 M at + ~ 0.85 Hydrothermal and sol-gel at +1.2 V NWs NaOH RHE 52 ZnO-TiO 2 Core-Shell 0.1 M Hydrothermal and ALD NWs Array KOH at at 0.1 V M ZnO@TiO 2 Core Hydrothermal and Sol-Gel NaOH Shell at TiO 2 -ZnO Core-Shell nano-heterostructures TiO 2 NBs-ZnO NRs Al-doped ZnO (AZO) NWs ZnFe 2 O 4 AZO NWs modified ZnO-TiO 2 Core-Shell NWs ZnO NWs Anodization and wet chemical method Hydrothermal and Solution Growth Hydrothermal and Solution Growth CVD 0.5 M 0.5 M 0.1 M 0.5 M at +1.0 V - 44 (Illuminated light intensity-100 mwcm -2 (AM 1.5), Reference electrode-ag/agcl, and Three-electrode assembly) at V (120 mwcm 2 ) at at at at at at at Present work S14